1. Field of the Invention
The invention relates to pistons for internal combustion engines and more particularly to pistons for diesel engines.
2. Review of the Prior Art
Since the fuel in a diesel engine is ignited by the temperature of air compressed in the cylinder prior to injection of the fuel, it is necessary, if combustion is to take place, to compress the air by a predetermined amount to ensure that the required temperature for fuel ignition is reached. The fuel is, however, injected before a piston in a cylinder of the engine reaches top dead centre and so the pressure in the cylinder continues to rise after the fuel has been injected.
In view of the high compression ratios used in diesel engines, the maximum cylinder pressure, reached, at or shortly after top dead centre, can be substantial. This peak pressure imposes loads on the piston which can damage bearings and reduce the effectiveness of lubrication. In addition, it can cause shock waves to pass through the engine block which can in turn cause cavitation in water cooling systems which leads to erosion of metal from the water side of the cylinders. The rate at which the pressure rises also causes fatigue and cracking in the piston and reduces the life of the gudgeon pin bosses. Further, the high rate of pressure rise is an important factor in the noise emission spectrum of diesel engines. The maximum pressure also determines the amount by which the air can be pressurised before entry into the cylinder and affects adversely the equipment for injecting the fuel. In addition, in certain cases, it makes the use of a heater necessary on starting the engine.